feat(assert): add assert_within_delta

[Chemaclass]

Summary

Adds assert_within_delta <expected> <actual> <delta> — asserts a numeric value is within a tolerance: |actual - expected| <= delta.

• Supports floating-point values via bashunit::math::calculate (bc, awk fallback).
• Handles negative differences (absolute value).
• Rejects non-numeric input with a clear failure.

assert_within_delta "3.14159" "3.14" "0.01"   # pass
assert_within_delta "100" "105" "3"           # fail (diff 5 > 3)

Tests

• test_successful_assert_within_delta
• test_successful_assert_within_delta_with_a_negative_difference
• test_successful_assert_within_delta_when_equal
• test_unsuccessful_assert_within_delta
• test_unsuccessful_assert_within_delta_with_a_non_numeric_value

make sa + make lint pass; full unit suite green under Bash 3.2 --strict --parallel --simple; bashunit doc snapshot regenerated.

Docs: docs/assertions.md. CHANGELOG updated under Unreleased → Added.

Closes #744